The Simbu serogroup of the genus Bunyavirus, family Bunyaviridae contains 25 viruses. Previous serological studies provided important information regarding some but not all of the relationships among Simbu serogroup viruses. This report describes the nucleotide sequence determination of the nucleocapsid (N) gene of the small genomic segment of 14 Simbu serogroup viruses and partial nucleotide sequence determination of the G2 glycoprotein-coding region (encoded by the medium RNA segment) of 19 viruses. The overall phylogeny of the Simbu serogroup inferred from analyses of the N gene was similar to that inferred from analyses of the G2 protein-coding region. Both analyses revealed that the Simbu serogroup viruses have evolved into at least five major phylogenetic lineages. In general, these phylogenetic lineages were consistent with the previous serological data, but provided a more detailed understanding of the relatedness amongst many viruses. In comparison to previous phylogenetic studies on the California and Bunyamwera serogroups of the Bunyavirus genus, the Simbu serogroup displays much larger genetic variation in the N gene (up to 40% amino acid sequence divergence).